Navy Cryptology and the Battle of Midway:Our Finest Hour

by LCDR Philip H. Jacobsen, USN-Ret

a special feature of the BATTLE OF MIDWAY ROUNDTABLE

(Editor’s note: the following is the text of an address given by Jacobsen to a gathering of Naval Security Group personnel at San Diego in 2000. It has been edited slightly for clarity and to better suit this format. —RR)

The Advent of U.S. Naval Cryptology

Although my part in the Battle of Midway was very small, I appreciate this opportunity to relate to you some of the more important achievements of my contemporary naval cryptologists that made the success of the Battle of Midway possible.As a current member of the Naval Security Group, you can take pride in the great accomplishments of your predecessors, not only related to the Battle of Midway but long before World War II as well as throughout World War II.

There are not many naval cryptologic veterans alive today that were involved in providing the communications intelligence information that gave our inferior forces on land, sea and especially in the air the equalizer of knowing the composition of enemy forces, and when and where those huge Japanese forces would attack U.S. territory under Admiral Yamamoto’s grandiose invasion plan.This crucial communications intelligence information, when combined with the heroic actions of fighting forces under the brilliant command of Admiral Nimitz, led to the great U.S. victory in the Battle of Midway.

We should keep in mind that intelligence itself does not win battles.However, I believe the lesson of the Battle of Midway is that good, solid intelligence can make the difference between winning and losing a crucial battle for our country.I hope you will keep this in mind in the future.

What was the genesis of the naval cryptologic success at the Battle of Midway?So much was involved in building up dedicated experts in all the various fields of cryptology that it is impossible to point to one single source.Credit must be given to many individuals who operated under difficult conditions, extremely limited budgets, and poor promotional opportunities.This relatively tiny group of dedicated individuals accomplished much in their efforts over the years to keep abreast of the growing force of the Japanese navy and their ever increasing communications security precautions.With the Japanese instigation of war with the U.S., this cadre of technical experts made it relatively easy to expand into a large organization and to immediately provide increasingly vital intelligence to
not only U.S. Navy operational forces but also to U.S. Army and Allied forces in the Southwest Pacific and Indian Ocean areas.

Attacking JN-25

Despite successes with prior Japanese naval and diplomatic codes, the high priority placed on the small group of naval cryptologists to provide decrypts of Japanese diplomatic communications precluded any significant decrypts of the current Japanese fleet code, JN-25B.Immediately after the Japanese attack on Pearl Harbor, Station HYPO in Hawaii under Commander Joseph J. Rochefort was given the authority to attack JN-25B.By early 1942, HYPO was producing some usable JN-25B decrypts.Station CAST at Corregidor, which was moved to Melbourne after the Philippines fell, and Station NEGAT in Washington soon followed with a number of important JN-25B decrypts.

HYPO first reported an offensive action in the “AK” or Hawaiian area which culminated in the ineffectual bombing of Oahu on the night of 4/5 March 1942.Rochefort determined that the long range Japanese seaplane was refueled by a submarine at the isolated island of French Frigate Shoals.This information would later play a vital part of the preparation for the Battle of Midway.

The Japanese Plan for Midway

The Japanese geographical designator “AF” began to appear in partially decrypted messages as early as 4 March 1942.On 13 March, Corregidor firmly identified “AF” as Midway.Melbourne and Washington confirmed that “AF” was Midway from subsequent decrypts, but for some unexplained reason Washington evaluated it as a communications designator, not a geographical designator even though Midway was obviously not a Japanese communications station.

Decrypts in late April by Melbourne and Hawaii showed intentions of hostile Japanese action at Dutch Harbor and Kodiak in the Alaskan area.

Beginning on 1 May, activity in Japan proper reflected preparations for both the Midway and Alaskan areas and provided detail of Japanese planning and the size of the forces committed to each objective.As the Japanese ships departed their anchorages, communications intelligence provided information on their future disposition.Both Melbourne and Hawaii reported the pairing of Japanese Carrier Divisions 1 and 2 for exercise activity in home waters on 3 and 12 May.In addition, HYPO provided a decrypted message of 7 May 1942 containing the complete agenda for an “aviation conference” on 16 May called by Vice Admiral Nagumo in Kagoshima, Kyushu.Also to be discussed was an “amphibious assault” and battle for “air superiority” together with a study of organizations for use in dive bombing, torpedo attacks, bombing, and strafing to wipe out local resistance.

For some time the status of Admiral Kondo’s powerful Second Fleet was clouded. Finally on 8 May 1942, HYPO correctly associated the carriers of the 1st Fleet with several important 2nd Fleet elements and warned of a possible creation of a strike force organization under Vice Admiral Nagumo, Commander 1st Air Fleet, consisting of CarDivs 1 and 2, CruDiv 8, two battleships from BatDiv 3, and other 2nd Fleet elements.These early correct conclusions gave a major advantage to the planners in the U.S. Pacific Fleet. They were reinforced by Melbourne on 9 May by a decrypt ordering destroyer screens for many of the capital ships in the Striking Force and revealing a sailing date from Sasebo of 21 May.

Troubles in Washington

On 14 May Admiral King directed Admiral Nimitz to declare a state of “Fleet Opposed Invasion” and gave Nimitz complete control of all military forces, including B-17s in the Hawaiian Islands.By 16 May Admirals King and Nimitz were in almost total agreement concerning Japanese intentions toward Midway and the Aleutians.However, this view was in sharp contrast to the confusion that reigned between OP-20-G (Station NEGAT) and War Plans staff under Admiral Richmond K. Turner. Turner placed some ridiculous restrictions on what Station NEGAT could report.

On 16 May, Nimitz ordered Admiral Halsey [Task Force 16 with USS Enterprise and Hornet] to return to Hawaii, indicating the Japanese would probably make simultaneous offensives against Port Moresby, Dutch Harbor, and Midway where the main striking force would be employed.

Two days later, all three navy cryptologic centers reported that the Strike Force’s attack would be from the northwest from N minus 2 days until N day, while Hawaii and Melbourne added that the attack would be launched from fifty miles northwest of AF.While this did not solve the attack timing problem completely, Nimitz immediately sent messages to Halsey and Fletcher [Task Force 17 with USS Yorktown] to expedite their return to Pearl Harbor as well as ordering submarine search activity off Midway to an area fifty miles northwest of the island.

An acrimonious relationship between Admiral Turner and his War Plans Division and OP-20-G continued, with Turner directing Commander Redman not to comment on certain intelligence evaluations and assume that Turner’s views were correct.The record suggests that the analysts in War Plans and OP-20-G were so engrossed in their own activities that they sometimes overlooked information concerning the Imperial Fleet readily available from translations in OP-20-GZ and the daily reports of the Pacific centers.

While the Pacific centers were convinced that the identity of AF was Midway because of its position in the “A” or American digraphs in the Japanese designator system, various persons at OP-20-G and in Washington thought it might be Johnston Island, Samoa, the U.S. West Coast or even Hawaii itself.HYPO was aware of this lack of agreement on AF in Washington.In order to rid themselves of this annoying backbiting, Rochefort approved a ruse that was probably thought up by Jasper Holmes, the author of Double Edged Secrets.Nimitz approved the message to be sent in the clear from Midway complaining of a water shortage.Rochefort let Melbourne make the first report of the decrypt from Tokyo Naval Intelligence advising of a “water shortage at AF.”Even the naysayers in Washington could not argue with this confirming evidence.

Stealing the Enemy’s Secrets

Additional information about a Japanese northern force prompted Nimitz to activate Task Force 8 under Admiral Robert A. Theobald.In spite of accompanying and subsequent accurate information about Japanese intentions in the Aleutians from decrypts, Theobald chose to treat such information as enemy deception and moved his forces out of the area to the Kodiak vicinity.That allowed the enemy to pound Dutch Harbor and occupy Kiska and Attu.

From information of Japanese successes in determining carrier movements simply by monitoring air to ground communications, Nimitz ordered Halsey and Fletcher to maintain radio silence, particularly among the aircraft when coming in to land.He also warned MacArthur that the Japanese were intercepting air-to-ground contacts between Port Moresby and allied planes.Nimitz also implemented a MacArthur suggestion that two or three U.S. vessels in the South Pacific conduct radio deception to create the impression that our carriers were remained in that area.

On 22 May, a Melbourne decrypt revealed the word “Midway” in a request for photographs of the island that had been “handed over to you.”Washington published a message from Nagumo to the 11th Air Fleet showing that his carriers had 33 aircraft on board that were destined to be the nucleus of land based aircraft in the new Japanese perimeter.Their loss was completely unnoted in accounts of Japanese carrier losses.

The 25th of May began with HYPO’s critical discovery of the Japanese date cipher.Now the U.S. possessed the means to determine the final ingredient of the Japanese plans—when the attack would take place.Application of this information allowed Rochefort to predict that the Japanese attack on the Aleutians would occur on 3 June and on Midway on 4 June.Despite objections from his staff, Nimitz decided to base his final timetable on these dates.Melbourne applied this date cipher information to older traffic and alerted the Pacific Fleet that on the 22nd of May CruDiv 8 and the battleships Kongo and Kirishima were scheduled to depart the Inland Sea of Japan.

Task Force 16 (Hornet and Enterprise) under Admiral Halsey returned to Pearl on the 26th and began a whirlwind of preparation for battle.The CINCPAC Bulletin of the 26th reported that the Northern Force had begun to depart Ominato and that all the Japanese carriers were probably at sea.Admiral Nimitz advised King how much he was dependent on communications intelligence and noted that they were only copying 60 percent of Japanese naval messages and only decrypting 40 percent of those copied.King attributed all of the Navy’s progress in the Pacific to the success it was having from timely information from Japanese naval codes.Without this information King said, “disaster is probable.”

Preparations for Battle

On the 27th of May, the Yorktown finally limped into port, showing the damage inflicted during the Coral Sea battle.This good news was offset by some bad news from Commander Rochefort’s center:a new underlying code (JN-25C) and additive cipher had been introduced that rendered unreadable almost all the texts of JN-25 messages from the 27th on.However, some previously originated messages were still readable including one from the 5th Fleet that contained tactical call signs for the Northern Force, its Strike Force, and the Occupation force for “AQ” and “AO” identified as Kiska and probably Attu.Again, Theobald refused to believe this intelligence and kept his force near Kodiak.Another prior message concerned the “Ichiki Detachment” to command
the 2nd Combined Landing Force, which was to occupy Midway’s Eastern Island.A third message revealed the intended use of civilian engineers captured on Wake Island to be used in the rebuilding of Midway.Additional warnings that the carriers were at sea were also published.

On 30 May, U.S. task force commanders were alerted by HYPO that direction finding had located three submarines in northern waters and one west of Midway.That day, the Yorktown (Task Force 17) slipped out of Pearl but was detected by the ComInt unit aboard the Yamato, Admiral Yamamoto’s flagship.However, due to radio silence restrictions, this information was not passed on to the Japanese carriers.NEGAT in Washington reported that the carrier Ryujo was at sea with the Northern Forces and that the Commander of the 6th Army Air Force was probably aboard the Akagi.

An old message produced the important information that fighter pilots from the carrier Zuikaku had been transferred to the Northern Force, ruling out the possibility that the Zuikaku could be called on to support either the Aleutian or Midway campaigns.Another message determined that major participants were called to a conference aboard the Akagi on the 26th, which meant they were still in port on that date.

Melbourne’s analysis of air activity in the Marshalls on 2 June led them to conclude that the Occupation Force was approaching the Marshalls.However, Admiral King’s headquarters report of that day contained serious errors.It estimated that BatDivs 2 and 1, CarDiv 4, and DesRon 3, parts of the Main Body, were still in the Bonins home waters area when in fact this force was approaching the western edge of the occluded front northwest of Midway.Perhaps, more importantly, the Office of Naval Intelligence chose this moment to report the presence of a fifth carrier, and identified the carrier as the Zuikaku.Fortunately, Admiral Nimitz and his intelligence staff had confidence in the information being generated by the centers in the Pacific, and this ONI estimate was not acted on or
repeated to the task forces off Midway.

Predictions Confirmed

As predicted by HYPO, the Japanese offensive against the Aleutians began on 3 June with the carriers attacking Dutch Harbor.Shortly thereafter, Midway notified Nimitz that the Japanese “Main Body” was sighted at 2100Z by a patrol plane bearing 261 degrees and a distance of 700 miles from Midway.After a second sighting of a smaller group of warships and cargo vessels, Nimitz advised that the forces sighted were the attack and occupation forces, not the main body.HYPO’s report of 3 June identified Admiral Yamamoto, CINC of the Combined Fleet as in overall command and correctly identified major commanders and functions of 2nd Fleet, 1st Air Fleet, and 5th Fleet.

Just after midnight on the morning of 4 June, Nimitz realized he had not yet advised the task forces how far the “Main Body” was from Midway.In addition to repeating earlier reports on its course and speed, he concluded it was now 574 miles from Midway.At 0604 Midway time, a reconnaissance plane from Midway spotted two Japanese carriers and their escorts and reported “many planes heading Midway” from 320 degrees, distance 150 miles. Less than a half hour later, Midway was attacked by Japanese carrier aircraft.

Nimitz was only able to muster 47 warships and 26 submarines against the Japanese fleet of 113 warships and 16 submarines.However, the U.S. was able to concentrate its forces at Midway with a slight advantage at the scene of the battle with three carriers, 22 escorts, 234 aircraft afloat and 110 at Midway versus four carriers, 17 escorts, 229 aircraft and 17 seaplanes for the Japanese.In addition, Admiral Nimitz and his task force commanders had advance knowledge of the identity of the Japanese objectives; virtually the entire Japanese Midway and Aleutian order of battle; the organization of the Midway forces into a Striking Force, Occupation Force, Invasion Force; the preliminary and final timetables of the Midway and Aleutian Striking Forces; the general direction from which each force would approach Midway, and the Midway Strike Force’s plan of attack.All of that information was supplied by communications intelligence in time to influence decisively the provisions of Admiral Nimitz’s Operation Plan 29-42.

In addition, luck was on the side of American forces in several key instances.Partly due to poor Midway bomber group sighting reports, two of the U.S. carrier aircraft groups [from Enterprise and Yorktown] were fortunate to locate the enemy carriers after changing their original course, while Hornet’s planes failed to make any contact.The late takeoff of the #4 search plane from the cruiser Tone prevented the Japanese from discovering the presence of U.S. carriers in time to make significant operational changes.

A Victory of Intelligence

The Americans lost only one carrier, one destroyer and 147 planes, while the enemy suffered the loss of four large carriers, all their aircraft, as well as one heavy cruiser and the damage to one heavy cruiser.These losses plus the rejection of the enemy invasion and occupation forces resulted in a huge victory for the U.S. Navy early in WWII.This great success after so much bad news from Pearl Harbor, the Philippines, and Southeast Asia was a great morale booster to the American people.

After the battles of Coral Sea, Midway and the Aleutians, the invaluable contributions made by communications intelligence were recognized by senior naval officials in Washington and Honolulu.In their words, communications intelligence had given the United States a “priceless advantage” over the Japanese.In few battles before or since would any navy possess an enemy’s order of battle, their plan of attack, and their timetable, all of which had been provided to the U.S. Navy’s high command by the communications intelligence units in Hawaii and Australia under the direction of Commander Joseph J. Rochefort and Lieutenant Rudolph Fabian, respectively.